Render is all gray, tried multiple things
You're getting a grey screen, because all your objects are set to be visible but not render. 

If you check the outliner, you'll see that your "Skybox" object, "Bola" object as well as the spheres parented to the armatures are all set to not render (the camera icon is greyed out).



For each item that should be rendering, click the camera icon beside each one in the outliner.